This invention relates to a profiled plastics bag closure strip and an adhesive bonding method.
Releasable bags of plastics material are well-known. These bags include a bag forming substrate and closure strips along the mouth of the bag. Each closure strip has one of a cooperating pair of male and female profiled closure elements. The principal method of making such material has been to extrude a tube which includes the substrate and integral closure strips and then to flatten the tube and to cut and weld in various directions so as to form the individual bags. In more recent times there have been developments directed towards the use of an adhesive for bonding independently extruded closure strips to a substrate. G.B. Pat. No. 1546433 describes a general method for the adhesive bonding of a closure strip to a substrate
In the adhesive bonding method a relatively thick closure strip is bonded to a relatively thin substrate and where a hot melt type of adhsive is used a substantial quantity of heat is required in order to ensure that proper adhesion of the closure strip to the substrate is obtained. In technical terms it is necessary to ensure that there is sufficient heat to "wet out" the closure strip or at least that portion thereof which is subject to contact with the adhesive to form the bond.
In practice it has been found to be difficult to achieve a satisfactory bond between two thermo-plastic components such as a substrate and a closure strip. It is believed that this difficulty arises at least in part from one or more of the variables within the bonding method and the products and apparatus concerned. The adhesive applicator system tends to provide a varying output of bonding material. The speed with which the thermo-plastic components pass the adhesive applicator system varies. These two factors lead to a variation in the mass of adhesive per length of the substrate and closure strip. There can be variations in the pressures applied to the substrate and closure strip during bonding. There can be variations in the thickness of the profile structure due to variations inherent in an extrusion process. These two factors can result in a greater or lesser quantity of adhesive being squeezed out from between the two components or even in the two components failing to make adhering contact.
The present invention is predicated on the realisation that an important part of an adhesive bonding method concerning the components in question is to provide a given thickness of the adhesive material over at least a part of the contacting surfaces of the strip and the substrate. This thickness ensures, when a hot melt type of adhesive is used, that there is a suitable quantity of heat per unit area of the closure strip to ensure that "wet out" of the strip is obtained at least over that part of its surface area designed to make adhering contact with the substrate.